There are some membrane-based separation processes with phase transformation. Membrane distillation (MD) is an example. The mass transfer driving force in the MD process is the vapor pressure difference across the microporous membrane. Hot and cold fluids respectively flow at both sides of membrane. In the hot side, the liquid with a low boiling point volatilizes and cross the porous hydrophobic membrane bulk and condenses at the cold side. Heat transfer exists at same time during mass transfer, which includes heat conduction due to the temperature difference between the hot and cold sides, and latent heat accompanied by vapor transfer. The gained output ratio (GOR) in MD process is low, usually less than 1. External heat exchangers were usually used to achieve heat recycle. For example, (LEE H Y, HE F, SONG L M, et al. Desalination with a cascade of cross-flow hollow fiber membrane distillation devices integrated with a heat exchanger [J]. AICHE Journal 54(7)(2011)1780-1795), and (LU S J, GAO Q J, WU C R, et al. Study on the process of decompressing and grading multi-effect membrane distillation. Journal of Tianjin Polytechnic University 32(2) (2013): 1-6). However, additional external heat exchangers increased the equipment cost, makes the system complex, and a little GOR increase.
Internal heat recovery was realized by inserting a heat recovery unit within a single MD module to increase GOR. In 1971, Henderyckx applied for a patent on the flat-shaped AGMD module with an internal heat recovery mechanism (U.S. Pat. No. 3,563,860). In the flat plate of the AGMD module, the condensate plate, the air gap and the hydrophobic porous membrane formed a “sandwich” form. Seawater was simultaneously directly to flow past the condensate plate in the counter direction and preheated by vapor at the air gap to absorb the latent heat of condensation. After additional heating, the preheated seawater was contacted with the membrane. Subsequently, the seawater was concentrated, and the latent heat was recovered in a module. In 1985, Gore et al. (U.S. Pat. No. 4,545,862) observed that the GOR reached as high as 11.0 for desalination with a spirally wound AGMD module.
In 1999, to develop a low-cost seawater desalination technology, a scientific institution in the Netherlands (TNO) presented a schematic of the countercurrent flow transmembrane evaporation module (AGMD) with latent heat recovery (GUIJT C M, RACZ I C HEUVENJW V, et al. Modelling of a trans-membrane evaporation module for desalination of seawater. Desalination 126(1999) 119-125). This module, later named the Memstill® process, consisted of several vertical microporous, hydrophobic hollow fibers with parallel cooling plates on both sides of the module separated by air gaps. The Memstill® process was patented in 2004 (U.S. Pat. No. 6,716,355B1). In 2005, the simulation results of a mathematical model of the Memstill® process showed that a high flux MD can be obtained at high feed temperatures, narrow gaps and thin membranes. For large air gaps of 3 mm, energy efficiencies of 85-90% were typically obtained, which were slightly below the theoretical values (95-98%) for a small heat loss to the surroundings. (GUIJT C M, MEINDERSMA G W, Reith T, et al. Air gap membrane distillation 2. Model validation and hollow fiber module performance analysis [J]. Separation and Purification Technology 43(3) (2005) 245-255). In 2006, Meindersmaa et al. invented a multi-effect flat-membrane module with heat recovery called the Memstil® system with a water ratio between 9-20 (MEINDERSMAA G W, GUIJT C M, DE HAAN A B. Desalination and water recycling by air gap membrane distillation. Desalination 187(1-3) (2006) 291-301).
In 2013, the Qin Group from Tianjin University used a self-made multi-effect membrane distillation module with high-efficiency internal heat recovery to conduct concentration studies on different concentrations of sodium chloride aqueous solution, and the GOR was up to 12.5 (YAO K, QIN Y, YUAN Y. A continuous-effect membrane distillation process based on hollow fiber AGMD module with internal latent-heat recovery. AICHE Journal 59(2013) 1278-1297). In the same year, Geng et al. designed a membrane distiller to concentrate high-concentration brines, which consisted mainly of hollow fiber porous membranes and hollow fiber dense tubes arranged in parallel and arranged with a partition between membranes and tubes. The thermal efficiency of membrane distillation process was effectively improved to 90% (CN 203155102 U). The GOR got to 6.44 (GENG H, HE Q, WU H, et al. Experimental study of hollow fiber AGMD modules with energy recovery for high saline water desalination [J]. Desalination 344(2014) 55-63). In 2015, Li et al. developed a module with an insulated tubular screen and hollow fiber membrane. The intertwined arrangement of hollow fiber membrane and hollow fiber condensing tubes promoted turbulence and increased the mass transfer and heat transfer coefficients in the boundary, and concentration and temperature polarization effects were weakened. The thermal efficiency of membrane was 94.3% and the GOR was 5.73 in seawater desalination. (LI B Y, WANG J Y, WANG J H, et al. New hollow fiber air gap membrane distillation for seawater desalination, Journal of chemical industry 66 (1) (2015) 149-156). In 2016, Liu et al. inserted a hollow fiber membrane into a capillary copper tube and invented a new double-tube air gap membrane module with a maximum GOR of 6.6 (LIU Z, GAO Q, LU X, et al. Study on the performance of double-pipe air gap membrane distillation module. Desalination 396(2016) 48-56).
Heat pipe is a new heat exchanger with high efficiency, as shown in FIG. 1. It is a closed metal tube without condensable gas. The inner surface of the tube is covered by a wick with capillary structure. The tube was filled with a condensable liquid as working fluid, which can penetrate into the wick by capillary force. When one end of the tube is heated, which is called evaporation end, where the working fluid absorbs heat and vaporizes. The resulting vapor flows to the other end, which is called cold end or condensation end, condenses and releases latent heat. Then the working fluid flows back to the evaporation end in the wick by capillary force. The evaporation-condensation process is repeated inside the heat pipe and the heat is continuously transferred from the evaporation end to the condensation end. Because the heat transfer coefficients in evaporation and condensation are far higher than the convective heat transfer coefficient and the flow resistance loss of vapor is low, the apparent thermal conductivity of heat pipe is hundreds of times of the best metal thermal conductor, it is also called thermal superconductor. Heat pipe is particularly efficient in gas-gas heat transfer process, where convection heat transfer coefficient is low. Furthermore, heat pipe is a closed pipe without moving parts, is resistant to wear, and basically doesn't require maintenance. Heat pipe can almost replace all the heat exchanger. In recent year, heat pipes are widely used to recover waste heat in boiler exhaust to preheat the combustion air.
In this invention, heat pipe replaces the dense membrane in the hollow fiber membrane module with latent heat recovery above-mentioned; and porous membranes are surrounded by heat pipes to form a membrane module with heat recovery. When this novel heat-pipe membrane module is used in a membrane process with phase transition or a high-temperature permeate, for example in the membrane distillation process, the vapor across the porous hydrophobic membrane acts as a heating medium to heat the evaporation end of the heat pipe; the cooling liquid acts as a cooling medium to cool the condensation end. In this way, the vapor generated in membrane distillation continuously heats the cold liquid and the latent heat of vapor is recovered. Since the thermal conductivity of the heat pipe is far higher than that of general polymer film, the heat recovery efficiency of this novel membrane module coupled with heat pipes will be far higher than the above-mentioned membrane module.
When recovering the heat of a high-temperature permeate, which acts as the heating medium, and other cold fluid may act as the cooling medium. In addition to coupling with hollow fiber membranes, heat pipes can also be coupled with other various types of membranes.